When I initially reforged it, I reforged the 155 expertise rating to 93 expertise and 62 mastery rating. However, on second thought I reforged the big chunk of parry rating to mastery instead, giving me 219 parry rating and 146 mastery rating.

Reforging the parry would give me +0.41% combat table coverage, however more block and less parry.

Originally I had the Tier ilvl 359 pants which gave me more combat table coverage as the expertise-reforged pants, more strength AND the 4 set bonus, so my only gain is the stamina and I feel that I have to reforge the parry if I want to upgrade compared to the tierlegs...

I take it you are not block capped, and you want combat table coverage more than you want to maximize your damage reduction?

If that's true, then yes, you absolutely want to reforge the parry to mastery instead of the expertise on your [Legguards of the Unseeing].

Without just trying them on, you can work out the relative math using the mastery/avoidance tables I posted a few weeks back.

Your parry is approximately 13.4%. Obviously it will vary a bit based on this choice, but that's close enough to find the correct row on the table. With 13.4% parry, it takes 2.788 parry rating to provide the same combat table coverage as 1 mastery rating. However, it takes just 1.115 parry rating to achieve the same total damage reduction as 1 mastery rating.

Thus, if you want to maximize your combat table coverage, 225 > 193 and you should reforge parry to mastery. If you want to maximize total damage reduction, 389 > 342 and you should reforge expertise to mastery.